A large part of the traffic load in future wireless communication systems is expected to originate from indoor users, for example from users in office buildings, cafés, shopping malls etc. Providing the indoor users with high bit-rate and spectrally efficient communication from outdoor base stations is challenging due to the penetration loss that is experienced by signals propagating through building walls. One well-known solution for enhancing the indoor coverage is to use outdoor-to-indoor repeaters. An outdoor-to-indoor repeater has a pick-up antenna on the outside of the building connected via a double-directional power amplifier to a donor antenna on the inside of the building.
Another well-known solution is to deploy pure indoor systems for example by deploying an indoor radio base station (RBS) and connect it to a distributed antenna system (DAS) where the antennas are also located indoors and close to the users. As an alternative to the use of DAS, leaky cables, for example coaxial cables, may be used.
Typical use cases for leaky cables are indoor deployments and along railway tunnels. Generally, a leaky cable may be constituted by a coaxial cable with slots or gaps along its entire length which enables the cable to radiate electromagnetic waves. Such a cable exhibits radiation properties different to a DAS, such as for example more uniform signal levels over the coverage area. A leaky cable can be used to both transmit and receive electromagnetic waves, i.e. it allows two-way communication. Indoor areas are typically rich scattering environments that allow Multiple Input Multiple Output (MIMO) communication.
Leaky cables are relatively expensive and cumbersome to install due to their high weight and stiff profile. In multi-stream applications such as MIMO communications, multiple cables need to be installed more or less in parallel which complicates the installation even more. MIMO communications are typically applicable indoors where high bit rates are sought after. Since a leaky cable leaks energy along its entire length, it has a high degree of attenuation per meter, which means that the signal to noise ratio (SNR) experienced by a user device located close to the end of the leaky cable is much less than if it was located at the beginning of the leaky cable. This leads to a very skew capacity distribution along the cable, which is very much undesired.
One way to combat this skewness is to introduce multiple radio frequency (RF) amplifiers or repeaters along the leaky cable, being arranged to amplify the signal propagating through the cable as described in for example EP 0442259. The major drawback is that such a solution is expensive and that a power source at each installation point is needed, which largely complicates the installation.
It is therefore a desire to provide an uncomplicated communication arrangement with a leaky cable where there is a more uniform capacity distribution along the cable than previously known such systems.